Aromatic hydrocarbon resins, obtained by reacting a polycyclic aromatic hydrocarbon having an alkylnaphthalene such as methylnaphthalene and/or a dialkylnaphthalene such as dimethylnaphthalene as main components with a paraformaldehyde in the presence of an aromatic monosulfonic acid, are conventionally well known as resins excellent in the compatibility with epoxy resins and the like and the solubility to organic solvents such as xylene (see Patent Document 1).
There are also well known methods of providing modified dimethylnaphthalene formaldehyde resins having high heat resistance by modifying dimethylnaphthalene formaldehyde resins with naphthols, phenols or the like (see Patent Documents 2 and 3).
It is known that under the usual reaction condition of producing such aromatic hydrocarbon formaldehyde resins, since diarylmethanes formed from two molecules of aromatic hydrocarbons and one molecule of formaldehyde are produced, and remain as unreacted components also after modification, the mechanical strength and the thermal decomposition resistance of cured products obtained from modified resins decrease. Then, there is made an attempt of suppressing the formation of the diarylmethanes by controlling the reaction condition (see Patent Document 4).